In a design flow for manufacturing semiconductor integrated circuits (IC), various methods and electronic design automation tools are incorporated to set up a desirable simulation environment. A variety of built-in device models and design rules are used to verify the performance of the design, such as the functionality, power, operation time, and die size. In addition, different models with different sets of parameters need to be prepared for addressing different applications. Among the model parameters, a heat-related parameter is utilized to simulate the influence of thermal sources on device functionality and performance. Then, a reasonable design margin is determined and reflected on the design IC by taking into account the thermal effect. Thus, functionality errors or performance degradation due to device heating is minimized in the device.